1. Field of the Invention
The present invention provides a normal-conducting photoelectron linear accelerator for producing a low-emittance electron beam from a photocathode that operates in ultra high vacuum and under high heat load.
2. Description of the Prior Art
A polarized electron linear accelerator based on a Plane-Wave-Transformer (PWT) design was the subject of a prior U.S. Pat. No. 6,744,226, in which a plurality of iris-loaded disks are suspended by water cooling rods (or pipes) that are connected to two endplates of a cylindrical radiofrequency (RF) cavity. The electric field pattern in the cylindrical PWT cavity is such that a TEM-like mode, resembling the plane wave in free space, is sustained in the region between the outer diameter of the disks and the inner wall of the cylindrical cavity, while a TM01-like mode is sustained on and near the axis of the standing-wave PWT cavity. Because the disk(s) are not attached to any other parts of the cavity than the supporting rods, the PWT has excellent vacuum properties including a large vacuum conductance in the paths from the photocathode that is located on the back endplate to the vacuum pumps located outside the cavity. A polarized electron beam is generated from a GaAs cathode located in the center of the back endplate of the cavity when a polarized laser beam is impinged upon it. Ultra high vacuum (UHV) can be accomplished with conventional ion pumps as well as non-evaporative getters (NEG). In the previous invention, a NEG film is sputtered onto the inner surface of the cavity wall. The presence of the NEG film on the RF cavity wall, however, reduces the Q-factor of the cavity. Also in said invention the NEG-lined cavity wall is not replaceable. As the NEG pumping becomes less effective over time, the entire cavity would have to be replaced. The cooling of the disks, rods, endplates and other elements in the PWT cavity that are exposed to RF heating during electron acceleration is accomplished by water flowing through internal channels inside the disks, rods and other elements. The flow rates are determined by the external pressure head and by resistances through the pipes and orifices as well as those in the internal channels of the disks and walls of the cavity. The flow rates are predominantly limited by the flow area inside the pipes and the sizes of orifices, which in turn limit the amount of heat that can be removed from the surfaces of the cavity that are exposed to RF. Such limitations can become problematic when a high heat load such as that required when long RF pulses, a high rep rate and/or high power RF are imposed on the PWT cavity. What is desired under such circumstances is an RF cavity that operates in a UHV environment with replaceable NEG elements and if possible, without the flow restriction imposed by the rods, orifices and disks.